1. Field of the Invention
The present invention relates to a method and apparatus for producing coatings using supersonic gas flow. More specifically, the present invention relates to a method and apparatus that applies coatings using a hybrid of plasma and cold-spray coating techniques.
2. Description of Related Art
Plasma thermal spray processes have been used in the coatings industry for many years. The plasma thermal spray process basically involves spraying molten or heat softened material onto a substrate to form a coating. Feedstock material, typically in powder form, is injected into a high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. Plasma guns can readily produce gas/plasma temperatures in excess of 6000 degrees C. The transfer of electrical energy to thermal energy is fairly efficient, with almost all of the losses occurring as waste heat taken out of the nozzle and electrode section as a result of the temperature and latency of the gas.
Although cold spray concepts first were patented at the turn of the 20th century, compared with plasma technology, commercial application of cold spray technology is a much more recent addition to the industry. Cold spray has the advantage of producing cold coatings with minimal heat input that can result in the coating material approaching the wrought properties of the source material itself. A key characteristic of a cold spray coating is the resulting compressive stress that exists in the coating. Classic thermal spray processes that produce coating as a result of melting or partial melting of the feedstock material result in tensile stresses as the coating cools and contracts. These tensile stresses often result in cracking of the coating, especially when producing thick layers or combining different layers of dissimilar materials.
Recent advances with plasma guns have made the feasibility of a hybrid process possible. For example, cascaded plasma guns enabled the practical extension of the plasma arc and the overall bore length to increase the gun operating voltage and thus the efficiency of the heat transfer to the gas. This has also led to the potential to separate the generation of plasma from gas dynamics. Also, incorporation of a step or discontinuity in the nozzle section of a plasma gun to plant the arc at the base of the nozzle has enable a separation of gas dynamics from the arc itself. It is then possible to utilize the generation of plasma as a thermal source separate from the nozzle as a method of gas acceleration. Another advance has been the design of an extended nozzle to permit the incorporation of a convergent/divergent section to the nozzle to accelerate the hot gas/plasma to supersonic velocities after the gas was heated by a plasma arc.
Supersonic velocities in plasma guns have been achieved in a number of previous designs. But the velocity in these designs has been limited to approximately Mach 1 in standard operating environments. Greater velocities have been achieved using designs that require costly process variations, such as operation in a vacuum or the use of large amounts of gas (e.g., helium) to achieve the higher velocities. In these previous cases the gas/particle temperatures were too high and often well in excess of the material melting temp. In addition, with the plasma process and even the high velocity oxygen fuel (HVOF) and cold spray processes, the operating efficiency and expected hardware life decreased rapidly as gas velocities increased. This is due to the nature of the design to accelerate the gas in conjunction with the heating of the gas using conventional plasma guns. Also, as gas flows and pressures are increased in a conventional plasma gun, the stability of the arc decreased and the arcs become more constricted.
Thus, in light of the factors described above, there remains a need in the art for a coating deposition process that achieves the beneficial compressive stress of cold spray coatings while providing a wider operating range of both particle velocities and temperatures than can be achieved by conventional cold spray or plasma techniques.